Wire grid antenna exhibiting luneberg lens properties



Jan, 9, 1968 P. JJSFERRAZZA 3,3 ,2

IRE GRID ANTENNA EXHIBITING LU'NBBERG LENS PROPERTIES Filed Jan. 25, 1965 2 Sheets-Sheet 1 I NVEN TOR.

' PETER J. SFERRAZZA A TOR/V5) WIRE GRID ANTENNA EXHIBITING LUNEZBERG LENS PROPERTIES Filed Jan. 25, 1965 Juli. 9. 1968 P. J. SFERRAZZA 2 Sheets-Sheet 2 RECEIVER INVENTOR. PE T51? J SFERRAZZA TORNEY United States Patent 3,363,251 WERE Gilli) ANTENNA EXHEBITING LUNEBERG LENS PRUPERTEES Peter I. Sferrazza, Huntington, N.Y., assignor to Sperry Rand Corporation, Great Neck, NFL, a corporation of Delaware Filed Jan. 25, 1965, Ser. No. 427,668 1t) Claims. (til. 343-754) This invention relates to antennas and more particularly to scanning lens antennas.

Scanning antennas are employed for transmission when a beam of radio frequency energy is to be directed successively over the elements of a given region or for reception when energy is to be accepted from successive elements in a given region.

One well-known scheme for providing scanning action involves the use of a Luneberg lens. In its basic form, this device comprises a dielectric sphere or disk in which the index of refraction varies in a specified fashion from the center to the edge of the body. Rays from a point source on the surface of the sphere or disk are focussed into a bundle of parallel rays emerging from the opposite side of the body. Conversely, a plane wave incident on one side of the body emerges from a single point on the opposite side of the body.

Although the Luneberg lens antenna has proven to 'be valuable, its use is somewhat limited by the fact that it is necessarily bulky and expensive to manufacture.

It is an object of the present invention to provide a lens antenna that is compact and easy to fabricate.

It is another object of the present invention to provide a lens antenna that can accept or provide energy throughout a wide angle.

It is still another object of the present invention to provide a lens antenna that can be used to provide rapid beam steering over a hemispherical space.

These and other objects are accomplished according to the principles of the present invention by providing a grid of interconnected transmission line segments in which a pattern of unequal delays is used to simulate the effects of refraction occurring in a conventional electromagnetic lens.

The principles and operation of the invention Will become more readily apparent by referring to the following description and the accompanying drawings.

FIG. la is a diagram illustrating the operation of a Luneberg lens,

FIG. 1b is a diagram illustrating the operation of a known Rinehart surface analog,

FIG. 2 is a diagram of a basic uniform grid pattern that may be used in practicing the invention,

FIGS. 3 and 4 are diagrams illustrating variations of the basic grid pattern that may be used in practicing the invention,

FIG. 5 is a diagram useful in explaining the operation of the invention,

FIG. 6 is a schematic diagram illustrating the manner in which the lens antenna may be used in a practical environment, and

FIG. 7 is a schematic diagram illustrating the manner in which the lens antenna may be used for scanning throughout a solid angle.

R. F. Rinehart in an article entitled: A Family of Designs for Rapid Scanning Radar Antennas, appearing on pp. 686-688 of the June 1952 Proceedings of the IRE, vol. 40, No. 6, describes a surface analogy of a Luneberg lens.

Rinehart recognized that a relationship exists between geodesics on a surface and ray propagation in a disk-like variable index-of-refraction medium. Rinehart then used this relationship to develop a surface analogy to a Luneice berg lens. Thus as illustrated in FIG. 1a, a plane Wave front 11, impinging on a Luneberg lens 13 is refracted so that the emerging energy is concentrated at a point P. FIG. 1b depicts a dome-like surface 15 symmetrical around an axis Z and proportioned according to Rineharts teaching. This surface permits energy to fiow along geodesics. The surface may be fitted with a brim portion 17 if desired so that energy arriving in a horizontal plane emerges in the same horizontal plane. A given element of geodesic arc length, ds, on the surface corresponds to an element of optical path length, dq, in the plane. The required index variations in the plane are known, therefore the arc length of the incremental elements necessary to determine the required surface can be calculated.

Rinehart further showed that a parallel conducting plate wave guide having this surface as a mean surface would possess the same focussing characteristics since radio frequency energy propagating in a parallel plate medium will flow along geodesics.

The present invention extends these principles so that a Luneberg lens analog may be constructed in a form of a fiat disk that is easy to assemble as well as compact and convenient to use.

Any transmission medium can be shaped to follow a Rinehart contour. This medium would then exhibit the Luneberg lens properties. The medium must however, be mono-refringent regardless of the direction of the proagation of the wave. That is, the phase velocity of the wave must be identical in all directions about a point or the angle of arrival or departure will be incorrect.

Consider, now, a flat grid of interconnected transmission lines as shown in FIG. 2. Energy may be supplied to the grid or received from the grid by means of peripheral terminals such as the terminals 19.

The transmission line segments between neighboring junctions are of approximately equal length and preferably much less than a quarter wavelength. One-sixteenth wavelength segments are presently preferred since the structure is then physically applicable to low frequency operation, although longer segments can be tolerated.

The segments may be made in any convenient transmission line medium. Coaxial lines may be used, for instance, and conventional four-terminal coaxial power dividers having a low VSWR may be used for the junctions. Similarly, printed circuit techniques or strip transmission lines may be used where preferred.

It can be shown mathematically that the phase velocity in such a grid is independent of the angle of incidence. Because of this, the structure effectively simulates a parallel plate transmission line.

In order to practice the invention, a regularly-spaced pattern is laid out on a Rinehart surface and a transmission line grid is then constructed in accordance with this pattern. Finally this transmission line grid is compressed axially into the form of a flat disk.

The regularly-spaced pattern of FIG. 2 will necessarily be slightly modified when fitted to the curved Rinehart surface. However, the same basic pattern will be maintained in this process.

Although the square grid pattern is presently preferred, numerous variations of this pattern are possible. Regardless of the pattern selected, however, the spacings between conductors should be maintained at less than one quarter wavelength. In general, a pattern which also permits neighboring transmission line segments to be approximately equal in length is desired.

FIG. 3, for instance, illustrates a polar form that may be used for the uniform grid pattern. This pattern, being essentially circular, can be more easily fitted to a Rinehart surface than the square pattern. The points 21, when applied to the surface, represent the points at which terminals can be placed to provide means for connecting the grid to utilization apparatus. The segments in this pattern are of approximately equal length,

Similarly, a rhombic pattern such as that of FIG. l may be used if desired. The points 23 indicate the placement of terminals for connecting this grid to util; ation apparatus. The segments are of aaproxiinately equal length.

FIG. illustrates the manner in which a transmission line grid is formed with the aid of a Rinehart surface 25.

Assume that the polar grid pattern of FIG. 3 is to be us 1. Only a cross section of the Rinehart surface has been depicted in FIG. 5 for ease of understanding. The transmission line grid 27 is draped over the Rinchart surface so that the Z axes of the grid and the su "ce coincide. A diamctric transmission line segment lies over the exposed surface of the cross sectional view and the annular lines 31 appear normal to the diametric line.

The transmission line grid is then compressed all into a flattened disk Assuming that flexible transr ission line is to be used, an outer transmission line segment will extend between the points 35 and 37, Because of the axial compression, this segment will be fold-ed back upon itself to a considerable extent. The intermedi re segment extending between the points 37 and 39 will he f a lesser degree whereas the inner c tween the points 39 and 41 will reap little folding.

With some transmission line media, it is more con venient to use known types of slow wave structures or meander lines rather than folding the outer segments as previously described.

FIG. 6 illustrates the use of the lens analog for r tion when scanning is to be performed in a single plane. The grid of transmission line elements is mounted in a dislolike enclosure 43. This disk typically has a die ter of 8 wavelengths. A receiver may be selectively switched to any one of the peripheral output terminals in the illustration, these terminals extend over one quadrant of the disk. Antenna elements 4-7 are connected to the remaining terminals. The resultant beam scan sector is onposite the output terminals 45, thus the lens permits scan between the positions indicated.

Scanning in a pair of orthogonal planes may be accomplished by stacking several disks as shown in FIG. 7. With this arrangement, scanning in the elevation plane is accomplished by switching the receiver or transmitter to appropriale peripheral terminals Scanning in the azimuth plane is accomplished by applying appropriate proic-rtions of power or attenuation to the various disks by analog techniques.

The beam width in the azimuth plane of FIG. 7 may be reduced by adding more disks to the stack. The basic scheme of FIG. 7 may be expanded to cover a full hemispherical scan by arranging stacks of disks in a spherical structure.

The basic system of FIG. 7 may be used in a monopulse system by squinting two receive beams for amplitude monopulse or by comparing a signal from one half the array with a signal from the other half of the array for pl :56 monopulse.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspoets.

What is claimed is:

1. An electromagnetic lens analogue comprising a grid of interconnected transmission line segments, said grid being proportioned to provide signal delays equivalent to the delays occurring along a Rinehart surface; and a plurality of terminals around the periphery of said grid, each of said terminals being connected to an outermost transmission line segment.

2. An electromagnetic lens analogue comprising a grid of interconnected transmission line segments, said grid being dimensioned so as to be conformable to a Rinehart surface; and a plurality of terminals around the periphery of said grid, each of said terminals being connected to an outermost transmission line segment.

An electromagnetic lens analogue comprising a transmission line grid formed from interconnected segments, said grid being constructed so that neighboring segments have substantially equal electrical lengths, said grid further having openings whose dimensions are determined by projecting a regularly spaced grid pattern from a Rinehart surface axially onto a transverse plane; and a pluralit of terminals around the periphery of said transmission line grid, each of said terminals being connected to an outermost transmission line segment.

An electromagnetic lens analogue comprising a grid of interconnected transmission line segments, said having openings which are smaller than one quarter wavelength at the lowest frequency to be encountered, said grid further being dimensioned by projecting a substantially uniformly spaced grid pattern laid out on a Rinehart surface axially onto a transverse plane; said transmission line grid further having neighboring segments of approximately equal electrical length; a plurality of terminals around the periphery of said grid; means for coupling a plurality of antenna elements to a first group of individual terminals along one portion of said grid; and means for coupling auxiliary apparatus to terminals opposite said first group.

5. An electromagnetic lens analogue comprising a grid of interconnected flexible transmission line segments, said grid being dimensioned so that neighboring segments are approximately equal in length; said grid being further dimensioned so as to be conformable to a Rinc hart surface; said grid still further being axially compressed into a flat disk whereby the outer segments are folded bacl; on themselves; and terminals on the peripheral segments of said grid for conecting utilization apparatus to the device.

6. An electromagnetic lens analogue comprising a transmission line grid; interconnected transmission line segments forming said grid, said segments being spaced at intervals less than one quarter wavelength at the lowest frequency to be encountered; said grid having physical dimensions equal to the axial projection of a regularly spaced grid pattern on a Rinehart surface onto a transverse plane; said elemcnts being formed from slow wave structures having electrical lengths squat to the electrical l ngths of the corresponding segments on the regularly spaced grid pattern; and terminals on the peripheral segments for connecting utilization apparatus to the device.

7'. An electromagnetic lens analogue comprising a non-uniform grid of interconnected transmission line segments, each of said segments having an electrical length determined by projecting a grid pattern with square openings axially from a Rinehart surface onto a transverse plane; a plurality of terminals around the periphery of said non-uniform grid, each of said terminals being connected to a different one of the outermost of said segmerits.

The device of claim '7 in which the openings in the uniform grid are less than oncsixtcenth of the wavelength at the highest frequency to be used.

9'. In combination: a plurality of non-uniform grids,

' each of said grids being formed from interconnected transmission line segments compressed into a flat disk, said disks being arranged as an axially aligned stack; each of said segments having an electrical length determined by projecting the corresponding segment of a regularly-spaced grid pattern on a Rinehart surface onto a transverse plane; a plurality of terminals around the periphery of each dish; means to couple antenna elements to the terminals in a first sector of each dish; and means to couple utilization apparatus to the terminals in a second sector of each disk.

16. In combination: a plurality of non-uniform grids, each of said grids being formed from interconnected transmission line segments compressed into a fiat disk, said disks being arranged as an axially aligned stack; each of said segments having an electrical length determined by projecting the corresponding segment of a regularly-spaced grid pattern on Rinehart surface onto a transverse plane; a plurality of terminals around the periphery of each disk; said disks being further positioned so that the corresponding terminals in the various disks are radially aligned; means to couple antenna elements References Cited UNITED STATES PATENTS 3,234,556 2/1966 Tanner 343-753 10 ELI LIEBERMAN, Primary Examiner. 

1. AN ELECTROMAGNETIC LENS ANALOGUE COMPRISING A GRID OF INTERCONNECTED TRANSMISSION LINE SEGMENTS, SAID GRID BEING PROPORTIONAL TO PROVIDE SIGNAL DELAYS EQUIVALENT TO THE DELAYS OCCURRING ALONG A RINEHART SURFACE; AND A 